Interest in fuel cell batteries as power sources for portable electronic devices has grown. A fuel cell is an electrochemical cell that uses materials from outside the cell as the active materials for the positive and negative electrode. Because a fuel cell does not have to contain all of the active materials used to generate electricity, the fuel cell can be made with a small volume relative to the amount of electrical energy produced compared to other types of batteries.
Fuel cells can be categorized according to the type of electrolyte used, typically one of five types: proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric-acid fuel cell (PAFC), solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC). Each of these types of fuel cell can use hydrogen and oxygen as the active materials of the fuel cell negative electrode (anode) and positive electrode (cathode), respectively. Hydrogen is oxidized at the negative electrode, and oxygen is reduced at the positive electrode. Ions pass through an electrically nonconductive, ion permeable separator and electrons pass through an external circuit to provide an electric current.
In some types of hydrogen fuel cells, hydrogen is formed from a hydrogen-containing fuel supplied to the negative electrode side of the fuel cell. In other types of hydrogen fuel cells, hydrogen gas is supplied to the fuel cell from a source outside the fuel cell.
A fuel cell system can include a fuel cell battery, including one or more fuel cells (e.g., a fuel cell stack), and a fuel source, such as a fuel tank or a hydrogen generator. Hydrogen generators that supply hydrogen gas to a fuel cell can be an integral part of a fuel cell system, or they can be removably coupled to the fuel cell system. A removable hydrogen generator can be replaced with another one when the hydrogen producing reactants have been consumed. Removable hydrogen generators can be disposable (intended for only a one-time use). Both removable and permanently installed hydrogen generators can be refillable (intended for use multiple times) to replace consumed reactant materials.
Hydrogen generators can produce hydrogen using a variety of starting materials and a variety of methods for initiating the release of hydrogen gas. Hydrogen gas can be evolved when a hydrogen containing material reacts. Examples of hydrogen-containing materials include liquid or gaseous hydrocarbons (such as methanol), hydrides (such as metal hydrides and chemical hydrides), alkali metal silicides, metal/silica gels, water, alcohols, dilute acids and organic fuels (such as N-ethylcarbazone and perhydrofluorene). A hydrogen containing compound can react with another reactant to produce hydrogen gas, when the reactants are mixed together, in the presence of a catalyst, heat or an acid, or a combination thereof. A hydrogen-containing material can be heated to release hydrogen gas, such as in a thermochemical decomposition reaction.
In selecting reactants for use in a hydrogen generator, consideration may be given to the following: (a) stability during long periods of time when the hydrogen generator is not in use, (b) ease of initiation of a hydrogen generating reaction or other hydrogen release, (c) the amount of energy that must be provided to sustain the release of hydrogen gas, (d) the maximum operating temperature required, and (e) the total volume of hydrogen that can be produced per unit of volume and per unit of mass of the hydrogen-containing material(s).
In order to provide hydrogen over a long period of time without developing a very high pressure within the hydrogen generator, it is desirable to generate the hydrogen on an as-needed basis. This can be accomplished by controlling the release of hydrogen gas from the hydrogen-containing material(s), such as by reacting only a limited quantity at a time. While this can be achieved by segregating the hydrogen-containing material into small quantities, it can add cost and complexity to the manufacturing process, and can require such things as thermal insulation of adjacent quantities of the material as well as more complicated controls.
An object of the present invention is to provide a hydrogen generator with one or more of the following features: capable of producing a large total volume of hydrogen gas per unit of mass and per unit of volume of the hydrogen generator, capable of controlling the release of hydrogen to provide hydrogen on an as needed basis without producing an excessive internal pressure, capable of operating at or below a desired maximum temperature, capable of replacing spent materials with fresh materials, long term durability and reliability, and capable of being manufactured easily and economically.